All-trans Retinoic acid (RA) and its analogs have emerged as dermatological agents and as potential cancer chemopreventive/chemotherapeutic agents. As a class, these compounds share teratogenic potential and the tendency to cause vitamin A toxicities. However, the O-acyl glucuronide metabolite of RA has been suggested to be a less toxic, active material. The synthetic retinoid N-(4-hydroxyphenyl) retinamide (4-HPR) shows some unique activity as a breast cancer chemopreventive with further reduced toxicity and teratogenicity. Our own studies with 4-HPR-O-glucuronide (4-HPROG) have shown it to be an even less toxic, more effective preventive and therapeutic for DMBA-induced rat mammary tumors relative to 4-HPR. However, these glucuronides as well as the retinamide 4-HPR are unstable to hydrolysis back to the parent retinoid making it unclear what the active forms of the molecules are. We have been synthesizing stable C-linked analogs of 4- HPR and 4-HPROG for evaluation as breast cancer chemopreventive/chemotherapeutic agents. A number of these analogs show increased potency preventing DMBA-induced tumors and further reduced toxicity relative to 4-HPR. Like 4-HPR, these analogs bind weakly to the known nuclear retinoid receptors and, unlike RA, cause apoptosis in tumor cells in a manner that is not inhibited by retinoid receptor antagonists, leaving their mechanism of action incompletely understood. The specific aims of our proposed continued research are: 1) to scale up production of our preferred analog (4-HBR, 4-HBRCG, or 4-HBRCglucose) based on a current chemotherapy/chemoprevention experiment underway, 2) to complete chemoprevention evaluation of the chosen analog in the DMBA-induced rat mammary tumor model, 3) to begin study of the chosen analog in a second (MMTV/ErbB2 mouse) mammary tumor model, 4) to conduct predictive teratology studies of the chosen analog(s), and 5) to explore mechanism of action issues with the chosen analog(s) focusing on our observations that 4-HPR and our analogs appear to induce ER stress. We will focus here on establishing the necessity of these phenomena for the actions of these compounds as well as determining molecular players upstream of these processes which trigger these events.